Four days after they said they had broken up a gang suspected of robbing grocery stores and fast-food outlets in the Baltimore area since fall, city police are trying to solve a new puzzle.

Police today announced the formation of a special robbery detail to try to capture the armed thieves.

Last night, five men -- two armed with shotguns and two with semiautomatic handguns -- robbed a supermarket in northwest Baltimore, police said.

And yesterday afternoon, four men -- one armed with a shotgun and two with semiautomatic handguns -- hit a supermarket in northeast Baltimore, police said.

Police are confused by these latest robberies.

"We don't know what we have, whether it's one large gang or several small gangs" who are copycats, said police spokesman Dennis Hill.

Under the plan announced today, teams of four or more officers -- backed by Special Weapons And Tactical (SWAT) units -- will patrol each of the city's nine districts in an effort to prevent future robberies or apprehend the heavily armed thieves.

Concentrating primarily at night, these motorized teams will be augmented by foot patrol officers. The foot officers will be working eight hours overtime in addition to their regular eight-hour shifts, Hill said.

Hill would not disclose details, such as the total number of officers to be used and the added overtime costs.

In last night's robbery, five men, one of whom showed no weapon, entered the Giant Food store in the 5900 block of Reisterstown Road about 7 p.m. and ordered about 80 customers and employees to lie on the floor, police said. One of the men stood guard by the doors.

"One walked right in and said, 'I want you,' " to the only unarmed security guard, said Mike Fisher, 31, a deli worker. The robber held a handgun to the guard's head and ordered him into the store's office.

Lt. Joseph Russell, of the Northwestern District, said the gunmen took an undisclosed amount of money from the manager's office and two cash registers.

"No one was hurt and no shots were fired," Russell said.

After the robbery, the gang fled -- two running east on nearby Menlo Drive and three north after crossing Menlo, witnesses told police.

All of the gang wore dark clothing and at least one wore a ski mask, witnesses said.

About two hours earlier, four men, one of whom appeared unarmed, robbed the Valu Food store at Frankford Avenue and Belair Road, Hill said. There were about 50 customers in the store, he said.

One of the gang, armed with a semiautomatic weapon, went to the manager's office, pointed the weapon at the manager's head dTC and forced him to open the store's safe, Hill said.

"Apparently the manager wasn't moving fast enough and he hit the manager in the head" with the butt of the weapon, Hill said. That caused the weapon to go off, the bullet piercing a carton of cigarettes. A male cashier also was struck in the head by one of the bandits.

Police described the gang as in their 20s; one wore a white-and-blue ski mask and two wore long coats.